1. Field of Invention
This invention relates to electronic circuits for driving the transmitter winding of an inductive position transducer.
2. Description of Related Art
Inductive position transducers are widely used to measure relative displacements between one or more receiver windings and one or more windings or disrupting elements that modulate the inductive coupling between the receiver windings and a transmitter winding. In various conventional inductive position transducers, such as those disclosed in U.S. Pat. No. 6,005,387 to Andermo et al. and 6,011,389 to Masreliez et al., each incorporated by reference herein in its entirety, a lower power, intermittent drive circuit is used to supply a time-varying drive signal to the transmitter windings. In the 389 and 387 patents, the intermittent drive circuit discharges a capacitor through the inductor formed by the transmitter winding. This causes the transmitter winding to xe2x80x9cringxe2x80x9d. That is, the current released by connecting the charged capacitor to ground through the inductor formed by the transmitter winding and a serially-connected resistor oscillates and exponentially decays.
This circuit provides a clean sinusoidal signal having a single fundamental frequency that is directly dependent on the inductance of the transmitter winding. However, to use this decaying ringing signal, the peak amplitude of the largest peak in the signal must be carefully sampled to be able to accurately determine the relative position between the receiver windings and the disrupting elements and/or coupling loops. Moreover, because the ringing circuit quickly decays, only a single sample can be taken of this signal each time the capacitor is charged and then subsequently discharged through the inductor formed by the transmitter winding.
In contrast, in various other conventional systems, the transmitter winding is continuously driven. U.S. Pat. No. 4,737,698 to McMullin et al. discloses a system that uses a continuously driven inductive transducer. For example, the 698 patent discloses a power oscillator that runs at a frequency of 10 kHz to 1 MHz. This low frequency range indicates that the load inductance on the power oscillator is large. As is well-known in the art, large load inductances, and therefore large load impedances, are easier to drive than inductive transducers having small inductances, and therefore small impedances.
As disclosed in the 698 patent, a single capacitor can be connected in parallel with the transmitter winding to form a resonant tank circuit that increases the impedance. This is shown, for example, in FIG. 9. However, the 698 patent indicates this is optional, suggesting that for the transmitter windings disclosed in the 698 patent, the impedance need not be specifically tuned to resonate at the oscillation frequency, and/or that inductance of the transmitter winding need not participate in determining the oscillation frequency. The 698 patent also discloses that the parallel capacitor is located at the transmitter winding.
However, the 698 patent does not provide any suggestion of the location of the power oscillator, implying that the location of the power oscillator is not critical. Since a power oscillator located remotely from the transmitter winding must drive relatively unpredictable wiring impedances in addition to the circuit elements at the transmitter winding, this again suggests that for the transmitter windings disclosed in the 698 patent, the impedance need not be specifically tuned to resonate at the oscillation frequency and/or that inductance of the transmitter winding need not participate in determining the oscillation frequency.
In yet other various conventional systems, the inductive position transducer is incorporated into a readhead, such as those used in hand-held calipers, linear scales and other position transducing systems that measure distances to relatively high accuracy and resolution. FIG. 7 shows a block diagram of the transducer, signal processing circuit and transmitter driver of one such conventional position transducer 600. As shown in FIG. 7, a program microcontroller 610, which includes program memory and RAM, a calibration memory 670 and a gate array 680 are connected to a data bus 695. The gate array 680 is connected to and controllably drives a transmitter driver 685. The transmitter driver 685 is connected to a dual-scale transducer 620 over a pair of drive signal lines 686 and 687.
The dual-scale transducer 620 includes a first scale having a first transmitter winding and a first set of receiver windings and a second scale having a second transmitter winding and a second set of receiver windings. The first set of receiver windings are connected over the signal lines 622 to an input multiplexer 630, while the second set of receiver windings are connected over the signal line 624 to the input multiplexer 630. The input multiplexer 630 selectively connects the first or second receiver windings to a synchronous demodulator 640 over a pair of signal lines 632 and 634. The synchronous demodulator 640 synchronously demodulates the induced signal in the first or second set of receiver windings generated by continuously driving the first or second transmitter winding. The synchronous demodulator 640 outputs the synchronously demodulated received signal over a signal line 642 to an amplifier and integrator 650.
The amplifier and integrator 650 amplifies the synchronously demodulated received signal and integrates it to generate a position signal corresponding to the relative position between the set of receiver windings used to generate the synchronously demodulated receiver signal and either or both of a set of disruptive elements or a set of coupling windings. The amplifier and integrator 650 outputs an amplified and integrated position signal over a signal line 652 to an analog-to-digital converter 660 that converts the analog signal to a digital signal. The digital signal is then output over the databus 650 to the microcontroller 610. The microcontroller 610 analyzes the digital signal to determine a relative position for the inductive position transducer 620.
This relative position is then output over the databus 695 to the gate array 680. The gate array 680 then outputs the position signal, either in quadrature form or as a numeric value, to the input/output interface 690. The input/output interface 690 then outputs the signals to a signal line 699, which can be connected to a display device for displaying the numeric value of the position signal or to a control system, such as a numerically-controlled machine tool, that uses the quadrature signals as control signals.
FIG. 8 shows one exemplary embodiment of a digital drive circuit 700 that imposes a square wave on an impedance-adjusted serially-connected inductive-capacitive circuit 720. In this case, the inductor of the serially-connected inductive-capacitive circuit 720 is formed by the transmitter winding 122 of the transducer 620. This is shown in FIG. 8 for a digital drive circuit that is used to drive the transmitter winding 122 of the transducer 620, using an oscillating power source 710 that is connected between ground 702 and the impedance-adjusted serially-connected inductive-capacitive circuit 720. In particular, the impedance-adjusted serially-connected inductive-capacitive circuit 720 comprises a capacitor 750 connected in series with the first transmitter winding 122 between the output of the oscillating power source 710 and ground 702. The digital drive circuit 700 shown in FIG. 8 relies on frequency discrimination provided by this impedance-adjusted serially-connected inductive-capacitive circuit 720 to convert the square wave imposed on the impedance-adjusted serially-connected inductive-capacitive circuit 720 into an approximate sine wave.
FIG. 9 shows a second exemplary embodiment of a digital drive circuit 700 that imposes a square wave on an impedance-adjusted parallel inductive-capacitive circuit 730. In this case, the inductor of the impedance-adjusted parallel inductive-capacitive circuit 730 is formed by the transmitter winding of the transducer 620. This is shown in FIG. 9 for a digital drive circuit that is used to drive a transmitter winding 122 of the transducer 620, using an oscillating power source 710 that is connected between ground 702 and the impedance-adjusted parallel inductive-capacitive circuit 730. In particular, the impedance-adjusted parallel inductive-capacitive circuit 730 comprises a capacitor 760 connected in parallel with the transmitter winding 122 between the output of the oscillating power source 710 and ground 702. The digital drive circuit 700 shown in FIG. 9 relies on frequency discrimination provided by this impedance-adjusted parallel inductive-capacitive circuit 730 to convert the square wave imposed on the impedance-adjusted parallel inductive-capacitive circuit into an approximate sine wave.
It should be appreciated that the conventional driver circuits, described above, and minor variations of these conventional driver circuits, have provided suitable design solutions that are sufficient for the available and anticipated inductive position transducers and their associated transmitter windings. The conventional driver circuits and the associated design solutions have not been considered problematic, when used in inductive position transducers. As a result, driver circuits of significantly wider utility and/or significant design advantages have not been known or available for inductive position transducers. However, to extend the economy, utility, and/or accuracy of inductive position transducers, and to enable the use of miniaturized transducers manufactured using advanced techniques, the inventors have recognized that new driver circuits, offering characteristics previously unknown in inductive position transducers, are required.
That is, the above-described conventional driver circuits for an inductive transducer have various problems which make them unsuitable when attempting to accurately measure extremely small displacements at extremely high resolution, and especially when using compact inductive transducers which exhibit relatively low transmitter winding impedance. For example, the design and description of the continuously-driven inductive transducer disclosed in the 698 patent suggests a relatively large impedance and, therefore, a relatively low operating frequency and/or sample rate.
In contrast, in many applications requiring measurement during motion, inductive position transducers are advantageously operated with a very high operating frequency and/or sample rate, especially when attempting to accurately measure extremely small increments of high-speed motion at extremely high resolution.
It should also be noted that, when the inductance of the drive winding does not participate in determining the oscillation frequency of the above-described conventional drive circuits, the transducer signal output is detrimentally reduced to the extent that the oscillation frequency of the transmitter winding does not coincide with the resonant frequency of the transmitter winding. Additionally, while the transmitter winding drive circuits disclosed in the 389 and 387 patents are suitable for inductive position transducers designed to measure small displacements at high resolutions, they are generally most applicable for intermittent operation at low speed, and /or for low-power applications. They cannot provide the performance advantages of continuously driven transmitter winding drive circuits at relatively higher speeds and/or resolutions.
Also, while the digital drive circuits shown in FIGS. 8 and 9 can be continuously driven, and are, in various exemplary embodiments, suitable for driving an inductive position transducer capable of measuring extremely small displacements at high resolution, the approximate sine wave generated in the transmitter windings by these digital drivers has a number of harmonic components that significantly degrade measurement accuracy, complicate the signal processing circuitry, and produce unnecessary radiated electromagnetic emissions that are detrimental to the environment and that complicate the transducer layout and packaging.
Furthermore, when measuring small displacements at extremely high resolutions using small or miniaturized inductive transducers, the available signal from the miniaturized transducers is inherently reduced by their small size. Therefore, in order to attain the desired signal-to-noise ratio in such miniaturized inductive position transducers, the signal through the transducer should be maximized to compensate for the small size of miniaturized inductive position transducers. However, in many cases, the transmitter winding of such miniature inductive position transducers has only a small inductance, and thus only a small impedance. Such small impedances are difficult to drive.
In order to solve this-problem, the inventors have eventually studied a class of circuits known in the field of RF circuit design as impedance transformers. However, the inventors have found that the impedance characteristics of the transmitter windings of practical and compact inductive position transducers are not characteristic of the problems conventionally studied and solved by impedance transformers in the field of practical RF circuit design. In particular, the inventors have found that many of the impedance transformer circuits developed in the field of RF circuit design are inappropriate or impractical to use in inductive position transducers, due to, for example, cost, size, or electrical interference problems.
In other cases, the inventors have found that with actual components, that is, non-ideal components, the impedance transformer circuits designed according to conventional principles of RF circuit design do not behave as conventionally predicted for the range of circuit characteristics associated with practical advanced inductive position transducers. Furthermore, the inventors have found that the combination of circuit parameters that significantly improves the performance of actual advanced inductive position transducers deviates from the solutions determined according to conventional principles in the field of RF circuit design.
Accordingly, this invention has been particularly developed to provide a desirable set of characteristics when applied to various practical inductive position transducers.
This invention provides a drive circuit for driving various inductive position transducers with enhanced efficiency and accuracy.
This invention further provides a drive circuit for driving low-impedance and/or miniaturized inductive position transducers with enhanced efficiency and accuracy.
This invention further provides a drive circuit for driving low-impedance and/or miniaturized inductive position transducers with enhanced efficiency and accuracy at high operating frequencies.
This invention separately provides a drive circuit for an inductive position transducer that generates a more pure sine wave.
This invention further provides a drive circuit for an inductive position transducer that uses a linear amplifier to generate the more pure sine wave.
This invention additionally provides a drive circuit for an inductive position transducer that uses the linear amplifier and an oscillator configuration to generate the more pure sine wave.
This invention separately provides a transmitter driver for an inductive position transducer that determines the oscillation frequency based on the transmitter winding inductance.
This invention further provides a driver circuit for an inductive position transducer that uses the transmitter winding as part of the resonator that determines the oscillation frequency.
This invention separately provides a driver circuit having at least two degrees of freedom for determining the operating characteristics of an inductive position transducer.
This invention further provides a transmitter driver for an inductive position transducer that uses two capacitors in the resonant circuit to provide at least two degrees of freedom.
This invention additionally provides a transmitter driver for an inductive position transducer that uses a resonant circuit having a first capacitor in series with the transmitter winding and a second capacitor in parallel with the serially-connected first capacitor and transmitter winding.
This invention separately provides a transmitter driver for an inductive position transducer that allows the transmitter voltage to exceed the power supply voltage.
In various exemplary embodiments of the transmitter windings driven according to this invention, the transmitter driver for the inductive position transducer includes, for each separate transmitter winding, at least one operational amplifier. In various exemplary embodiments, a first feedback loop between the output of the operational amplifier and an inverting input of the operational amplifier is provided to provide a bias and set the gain for the operational amplifier. A second feedback loop, including the resonator circuit that includes the transmitter winding, is formed between the output of the operational amplifier and the non-inverting input of the operational amplifier.
In various other exemplary embodiments, the transmitter driver according to this invention includes a single-ended operational amplifier-based oscillator. In various other exemplary embodiments, for each transmitter winding, the transmitter driver includes a double-ended operational-amplifier based oscillator. In still other exemplary embodiments, the transmitter driver includes, for each transmitter winding, a digital driver. In all these exemplary embodiments, the resonant circuit including the transmitter winding includes a first capacitor connected in series with the inductor provided by the transmitter winding and a second capacitor connected in parallel to the serially-connected first capacitor and transmitter winding.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the transmitter winding driver according to this invention.